Nobel Prize in Physics 2019: Modern Cosmology and Exoplanets
M.V.N. Murthy, Chennai

The 2019 Nobel prize has been announced. One half of the prize is awarded to James Peebles for "theoretical discoveries in physical cosmology" and the other half is shared equally by Michel Mayer and Didier Queloz for the "discovery of an exoplanet orbiting a solar type star". James Peebles was born in Winnipeg, Canada in 1935 and is presently Albert Einstein Professor of Science at Princeton University, USA. 

The Big Bang
In 1912 Vesto Slipher observed that the light from distant galaxies was red-shifted (more about this later). This was confirmed by Edwin Hubble in 1929. The red-shift simply means that the remote galaxies are moving away from us. That is, the Universe is expanding for ever since its birth. Working backwards in time this obviously means that the Universe must have started extremely small and in a very hot state. The moment at which it all started is called the Big Bang, a name coined by Fred Hoyle.

Modern Cosmology
The real change came about in 1964 when Arno Penzias and Robert Wilson (Nobel Prize- 1978) were trying to study radio waves. But they could not remove the constant hum that their antenna picked up from every direction, night and day, no matter how much they tried. After many checks of their instruments, they concluded that this was not instrument noise as we often hear in a radio, but some thing coming from the space all around.
The breakthrough was provided when James Peebles immediately interpreted this hum as due to the radiation left-over from the Big bang. Today it is called the cosmic background radiation. Thus began the era of Modern Cosmology which is now almost entirely based on observations.

Evolution of the Universe
Today we have a deeper understanding of how the Universe evolved from the Big Bang into what it is today.
The Universe began around 13.7 billion years ago in the Big Bang. In the beginning everything including gravity is in the quantum regime and we do not understand this phase that well even today.
Immediately after this phase, the Universe underwent a rapid, exponential expansion. That is, the size of the Universe became exponentially larger. This is called the inflationary expansion of the Universe.
The radiation at extremely high temperature interacts to produce a soup of elementary particles like quarks (which will eventually make up protons and neutrons in the nucleus of atoms) and leptons (electrons for example). As the universe expands the soup cools so that the quarks combine to form nucleons- protons and neutrons. These then combine to form first the nuclei and then neutral atoms by capturing electrons.
Once the neutral matter is formed, about 375,000 years after the Big Bang, the radiation that is ever present decouples from ordinary neutral atomic matter and forms a permanent background. This is the Cosmic Microwave Background Radiation (CMBR) that Penzias and Wilson detected and James Peebles interpreted.
The temperature at which this radiation decoupled was still very high but over time as the Universe expanded, it cooed to its present temperature of 3 degree Kelvin or about -270 degree Celsius.
The neutral matter combines to form matter of all sizes and shapes like the stars, galaxies and every thing that we observe through telescopes. This is summarised in the schematic figure.

More on the Cosmic Background Radiation
The observation of CMBR opened up a wealth of information from which we are now able to reconstruct much of the history of the universe. While the CMBR is more or less uniform in all directions, there are subtle variations or ripples in the radiation. But for these ripples, the universe would have been a ball of uniform distribution of matter and space. But the space is full of clumps of matter; these are the stars, galaxies, clusters of galaxies etc. The ripples therefore reveal the slight variations that survives from the early universe: whereever there was more density, stars and galaxies have formed.
Now, scientists have observed these tiny non-uniformities in the background radiation with increasing precision. We are now able to conclude that nearly 95 percent of the universe is almost invisible to us. All the matter that is visible to us is just above five percent, that is the matter that constitutes all the stars, planets, mountains and seas, and all living things.

Dark Matter
About 25 percent of the rest is known as Dark Matter whose existence is known to us through its gravitational effect on the motion of stars and galaxies. The presence of dark matter is now well established through its gravitational effect on the motion of stars and galaxies. It is called dark since it does not emit any light (or em radiation). Peebles noted that if dark matter is made up of particles like our ordinary matter (electrons, protons, neutrons), then these particles must be at least ten times heavier than proton or neutron and probably much more. This is now called the cold dark matter. We still do not understand this very well.

Dark Energy
The rest, about 70 percent, which makes up the Universe, is callee dark energy. Very little is known or understood about this, but it is expected to drive the acceleration of the universe as established through observations in 1998 by Perlmutter, Schmidt and Riess (Nobel Prize 2011).
James Peebles is considered one of the founders of Modern Cosmology. Modern Cosmology is now an evidence-based science. Every step of our understanding of the universe and our place in it, has contributions from James Peebles. Unlike many other Nobel Prizes, this years prize honours contributions of Peebles made over a period of more than four decades, much like the prize given to scientist S. Chandrasekhar earlier. 

Exoplanets
Half the Nobel prize in 2019 is equally shared between Michel Mayor and Didier Queloz for their discovery of Exoplanets orbiting a star similar to our own sun. Michel Mayor was born in 1942 in Lausanne, Switzerland and is a professor at University of Geneva in Switzerland. Didier Queloz was born in 1966 in Geneva, Switzerland and is a professor at University of Geneva and also at University of Cambridge, UK.
There are about 10^{21} (one billion trillion) stars in the universe that is accessible to us for viewing. In fact there may be many more but we will never know since our observation is limited to a size of about 13.7 billion light years from Earth. Remember one light year is the distance travelled by light in a year. Given that there are such a large number of stars, is it possible that sun is the only star that has a planetary system? 
Any reasonable answer, based on chance or probability, would be NO; there must be other star systems with planets. Such planets are called Exoplanets, that is planets outside our solar system. So we already have the answer to the question. So what is the big deal about this year's Nobel prize? 

Difficulty of observation
Any object in the sky (stars, galaxies) is observed, using telescopes of various kinds, by the light it emits.
Planets do not emit light or glow of their own; they merely reflect the light from nearby stars. For example, we see other planets (also comets) in the solar system since these planets scatter the light from the sun. Farther the planet, the fainter it becomes. Ultimately it becomes too faint to be observable over large distances (say of the order of light years) against the background of light emitted by stars themselves.

The radial velocity method
So how does one observe exoplanets? It requires sophisticated and precise methods to observe these exoplanets which is what Mayer and Queloz did. They used a method called the radial velocity method.
Suppose there is a planet orbiting a star (like our solar system). Not only does the stars gravitational force keeps the planet in the orbit (remember Kepler), but the gravitational pull of the planet also affects the motion of the mother star. In fact both the star and the planet revolve around their common centre of gravity as shown in the figure.
Now here is the key: Even though the planet itself is not observable, its presence moves the star in a regular motion. For an observer looking through the telescope from earth, it would appear as if the star is moving towards us for half the time of revolution and moving away for the other half the time. This motion of the star, which to an observer on earth appears as simple motion either towards or away (radial motion) causes what is called Doppler effect (see Box).

BOX ON Doppler Effect
Doppler effect is actually familiar to all of us. If you are standing on a road and a truck is coming towards blaring the horn, you will actually hear a sound of higher frequency (shriller) than the actual frequency of the horn (when it is not in motion). Similarly when it is moving away the frequency of the sound is reduced. This is true of sound waves and also of light waves. If a star is moving towards the observer on earth then the frequency of light is shifted to higher frequency (called the blue shift) and if it is moving away from the observer then it is shifted to lower frequency (called the red shift). As a result of the Doppler effect, the colour of light emitted by the star keeps changing during its revolution around the center of gravity of star and planet system. This fact was used by the Nobel prize winners this year to identify the presence of an exoplanet.
END OF BOX
It took a long time to identify the first exoplanet which was done finally in the year 1995.
So what were the challenges:
. The first challenge is to identify the region in the sky where there likely to be planetary systems. Mayor and Queloz identified the star 51 Pegasi which is about 50 light years away from us. The first exoplanet is therefore called 51-Pegasi-b.
. The next challenge is to have a star which has a revolution period (period of radial velocity) which is not too long so that many observations can be made. 51-Pegasi-b takes about four days to complete its orbit, which means it is very close to the host star.
. The planet must have a bigger size to have sufficient impact on the stars radial motion. 51-Pegasi-b is indirectly estimated to be as large as our own Jupiter.
. The biggest challenge is measuring the changes in the frequency which are extremely small since radial velocities are extremely low. In our own solar system, the effect of a large planet like Jupiter is to cause the Sun to move at about 12 m/s around their common centre of gravity. Even this is small, but astronomers have sophisticated spectrographs to observe the effect of such small speeds.
At the end of meeting all these challenges, Michael Mayor and Didier Queloz identified for the first time in 1995 that they have discovered "an exoplanet orbiting a solar type star". They used numerous new technologies, like optical fibers that could carry the star light to the spectrograph without distortions, better digital image sensor, increased light sensitivity etc. This discovery was immediately confirmed by other groups of astronomers. Just a few months later two more exoplanets were found. Now we know of several hundreds of new planets and planetary systems discovered by not only telescopes on earth but also space bound telescopes. New methods are being devised which are better than the radial velocity method.
Now that we know there are exoplanets, or probably even exo-solar systems, is there life out there in space? What do you think?